1. Field of the Invention
The present invention relates to an information transmission method for reporting on-road location information such as traffic jams and accidents in a traffic information provision system, and in particular to such a method that can correctly report a location on a digital map.
2. Description of the Related Art
In recent years, the number of vehicles that have car-mounted navigation apparatus has been increasing rapidly. The car-mounted navigation system has a digital map database and is capable of displaying the map around the vehicle on a screen based on the latitude/longitude data received by a GPS receiver as well as displaying the travel locus and the result of search for a route to the destination on the map.
In Japan, digital map databases are prepared by several companies. The problem is that map data contains errors inherent in the reduced-scale maps. Some of the maps on the market drawn on the scale of 1:25000 contains errors of about 50 meters depending on the location, and the errors vary with digital maps available from those companies.
Latitude/Longitude data obtained from a GPS receiver may contain errors of several tens of meters.
Some models of car-mounted navigation apparatus on the market receive traffic jam information and accident information from a traffic information provision system and display the traffic jam and accident locations on the map or performs route search using such information as additional search conditions.
In a prior traffic information provision system, as shown in FIG. 8, traffic information is provided from a traffic information collecting center 71 that has local jurisdiction over an area to an information delivery center 72. Traffic information edited for each transmission medium including FM broadcasts, road beacons and cell phones is transmitted via respective media.
The traffic information collecting center 71 exchanges traffic information with a traffic information collecting center 78 in other areas to collect traffic information in a wide service area including the peripheral areas.
In the traffic information, for example, in case latitude/longitude data on the location is presented alone in order to report a traffic jam location or accident location, various errors are contained depending on the kind of digital map database of the car-mounted navigation apparatus as mentioned earlier. Thus, different locations on the road could be recognized as the accident location by the car-mounted navigation apparatus retaining a digital map database from Company A and the car-mounted navigation apparatus retaining a digital map database from Company B.
In order to alleviate incorrect information transmission, as shown in FIG. 9A, crossings a, b on a road network are assumed as nodes, and a road c connecting the nodes as a link. Each node is assigned a node number uniquely representing the node (a=1111, b=3333). Each link is assigned a link number uniquely representing the link (c=11113333). The node numbers and link numbers thus assigned are stored in the digital map database of each company, in correspondence with each crossing and road.
Traffic information represents a point on the road by first specifying a link number then the distance in meters from the start of the link. For example, a representation “Location 200 meters from the start of the road with link number=11113333” allows the user to obtain the location P on the same road by tracing the road with link number=11113333 as far as 200 meters from the node with node number 1111, irrespective of what digital map data is used by the car-mounted navigation system in question.
Node numbers and link numbers defined on a road network must be changed to new numbers in case a road d is constructed or a road is modified as shown in FIG. 9B. Such modifications to node numbers and link numbers require updating of digital map data from each company.
Construction of and modification to a road continue into the future. As long as on-road location representation is made using node numbers and link numbers, maintenance of digital map databases is permanently accompanied by huge workload and resulting costs.
The information delivery center must send information that complies with past node numbers and link numbers for a predetermined period, for example three to five years, as well as the information of the latest year, in order to support navigation systems sold in the past. This causes huge maintenance workload.
In order to solve such problems, the inventor of the invention has proposed a location information transmission method that can report a location on a digital map without excessive maintenance workload (JP2001-041757 A).
In this location information transmission method, an information provider transmits on-road location information by using road shape data consisting of string of coordinates representing the road shape of a road section including the on-road location having a predetermined length and relative data indicating the on-road location in the road section. A party receiving the on-road location information performs shape matching to identify the road section on the digital map and uses relative data to identify the on-road location in this road section.
For example, in case a traffic jam has occurred in the section A to B on the road shown in FIG. 6, the road shape of the road including the section is displayed in a string of coordinates consisting of 601 points in 5-meter intervals, P0 (x0, y0), P1(x1, y1), . . . , p600(x600, y600). Here, xk and yk are latitude and longitude data of the on-road point Pk acquired from the digital map database retained by the information provider. Further, the distance l1 from the point P0(x0, y0) to the traffic jam start point A and the distance l2 from the point P0(x0, y0) to the traffic jam end point B are obtained and the road shape data: (x0, y0) (x1, y1), . . . , (x600, y600) and traffic jam distance data: l1 to l2 m are generated as location information. This location information is converted to a predetermined transmission format and sent to the receiving party.
The receiving party that has received this information performs shape matching of map data in the digital map database retained by the receiving party and the received map shape data, and identifies the map shape on its own digital map. The receiving party identifies the traffic jam section to be displayed on its digital map based on the distance data from the start point of the road section.
FIG. 7 shows an example of shape matching.
Roads Q, R included within the error range around the P0 (x0, y0) point of map data read from the digital map database 7 are selected as candidates.
Then, locations Q0, R0 on each candidate road closest to P0 (x0, y0) are obtained and distance P0-Q0 and distance P0-R0 are calculated.
This operation is carried out for each point P1(x1, y1), . . . , P600(x600, y600) and the road section where the summation value of the root mean square of the distances from each point P0, P1, . . . , p600 is smallest is obtained.
Then the section l1-l2 meters from the start point of the road section is identified as a traffic jam section.
In this way, it is possible for the receiving party to identify an on-road location without defining node numbers or link numbers, by providing road shape data representing a road shape using a string of coordinates. This location information can be used when traffic information is exchanged between traffic information collecting centers or when traffic information is provided from an FM station or road beacon.
This method is disadvantageous in that it requires location information on a great number of points on the road to be transmitted in order to correctly report the road shape and is consequently accompanied by huge transmit data. Reducing the number of points on the target road could invite erroneous matching at the receiving party.